The invention relates generally to methods of improving the performance of MNOS structures and more particularly pertains to a method for reducing the density of electronic trapping states and fixed insulator charge in an MNOS structure with a thin oxide layer.
There is a growing class of applications for solidstate electronic devices, such as Charge-Coupled-Devices (CCD) and Field-Effect Transistors (FET), that require operability at cryogenic temperatures and tolerance to ionizing radiation. Such applications include the use of CCDs in conjunction with large area infrared photodetector arrays for imaging and signal processing. Another application is the use of solid state electronic devices in outer space.
In order to achieve radiation hardening and operability at cryogenic temperatues an MNOS (metal-silicon nitride-silicon dioxide-semiconductor) dual dielectric insulator structure is often used in place of pure silicon dioxide as the gate insulator in electronic MOS (metal-oxide-semiconductor) devices such as FETs and CCDs (the "gate" insulator refers to the insulator in the active regions of the devices such as the FET gate).
A nitride/oxide insulator structure utilized as an active gate insulator in an MOS device is described in the paper by M. C. Peckerar et al. entitled "Hydrogen Annealed Nitride/Oxide Dielectric Structures for Radiation Hardness", IEEE Transactions on Nuclear Science, Vol. NS-27, No. 6, Dec. 1980. In that article superior radiation tolerance of nitride compared with oxide is discussed along with the need for a thin oxide layer between the nitride and silicon regions to prevent memory effects. The authors found that high temperature post-dielectric processing degraded the performance of the gate due to the introduction of electron trapping states and fixed insulator charge in the oxide layer. However, they discovered that a high temperature hydrogen anneal reduced the interface density states to a value of about 1.times.10.sup.10 states/cm.sup.2 eV. This high temperature hydrogen anneal includes the steps of exposing the surface of the MNOS structure to hydrogen gas (H.sub.2) and heating the structure to a predetermined temperature.
It is well established that the diffusion rate of hydrogen is substantially higher in silicon dioxide compared to silicon nitride at typical annealing temperatures. Therefore, in MNOS structures, it has been shown that the hydrogen enters the oxide layer through vent holes in the nitride layer during the high temperature anneal. These vent holes, or other openings, must be formed in the nitride layer prior to the high temperature hydrogen anneal. The hydrogen then diffuses laterally into the gate region, and chemically reacts at the oxide-silicon interface. Thus, a very high annealing temperature (typically 650.degree.-1000.degree. C.) is required to obtain lateral diffusion of hydrogen through the oxide layer in MNOS structures when the layer is very thin (.ltoreq.100 .ANG.) as is required for radiation-tolerant devices.
This high annealing temperature, required by existing techniques, has several disadvantages. First, the heating of the structure increases the fixed insulator charge thereby degrading device performance. Second, the aluminum gate contact must be deposited after the hydrogen anneal since the aluminum can only tolerate temperatures up to 500.degree. C. However, aluminum deposition increases surface charge density. Finally, the density of electronic states has not been reduced below the level originally present before high temperature post dielectric processing.